Hydrogen sulphide (H2S) is a flammable, irritating, corrosive, typically bad-smelling and extremely toxic gas. Toxicity of the hydrogen sulphide is comparable with hydrogen cyanide, which is considered as a broad-spectrum poison. Hydrogen sulphide can affect different parts and systems such as skin, eyes and throat in the human body, depress the nervous system and eventually cause death. Hydrogen sulphide occurs naturally in the environment, but ultra low levels can be tolerated because the human body can possess a number of enzymes that are able to perform the conversion through oxidation of hydrogen sulphide to sulphate.
It is important to continually sense the hydrogen sulphide to provide safeguards for the employees who work in areas like petrochemical and fuel refinery industry where hydrogen sulphide exists. The detection of hydrogen sulphide is also very beneficial for the biomedical field, especially for determination of H2S content in mouth air and for diagnosis in dentistry. Semiconductor oxides play a significant role for H2S sensing. Tin dioxide-based materials such as pure SnO2, CuO—SnO2 and SnO2—Ag2O can easily sense H2S in air. Copper oxide is a best promoter for the SnO2-based hydrogen sulphide sensors. However, such sensors exhibit the maximum sensitivity at elevated temperatures, (i.e. around 150° C.). At this elevated temperature, irreversible reactions can take place between the gas and the sensing layer, which affects the long-term stability of the sensor.
Some prior art gas sensors utilize a thin solid film on a SAW/BAW device to overcome the aforementioned drawbacks. In such gas sensors, gas molecules are adsorbed onto the surface of solid film due to the interactions like hydrogen bond, electrostatic, pi-pi stacking, Van Der Waals and host-guest relationships. Therefore, the propagation velocity of the SAW/BAW acoustic waves can be alerted and eventually a shift in the phase or resonance frequency of SAW/BAW devices is induced, as a function of the gas. Sputtered inorganic film based on activated tungsten trioxide materials, (e.g. pure tungsten trioxide, doped tungsten trioxide with iridium, gold and palladium), can be developed to form a sensitive film for hydrogen sulphide detection. Such thin films exhibit a good sensitivity toward hydrogen sulphide, but unfortunately the temperature still remains too high, (i.e. around 130° C.).
Furthermore, single-wall and double-wall carbon nanotubes, well known as inert molecules, have been utilized to form the sensing film, but the manipulation of carbon nanotubes exhibits important disadvantages such as low solubility and difficult chemical and physical processing. In addition, covalent and noncovalent functionalization of the carbon nanotubes diminishes the inertness property. The most popular covalent functionalization involves sonication of carbon nanotubes in a mixture of concentrated nitric acid/sulphuric acid, which oxidizes the parent molecule and introduces different groups onto the surface of carbon nanotubes such as carboxyl (—COOH), hydroxyl (—OH) and carbonyl (—C═O). Also, amino carbon nanotubes are available through the intermediate of Curtius transposition or Hoffmann degradation. Therefore, such single-wall and multi-wall carbon nanotubes are functionalized with mercapto groups (—SH) to overcome such disadvantages.
In addition, the functionalization of the molecular architecture of carbon nanotubes has been developed for the synthesis of CNT—CO—NH—(CH2)11-SH. This derivatization involves sonication of carbon nanotubes in nitric acid/sulphuric acid mixture, the addition of thionil chloride to convert carboxylic groups to the corresponding acid chloride, and treatment with α, ω mercapto-amine bifunctional compound to produce the alkanethiol. It has been shown that the mercapto-group is located at the end of alkylic chain with eleven carbon atoms, and that the mercapto-amide groups form a bridge between parent carbon nanotube and alkanethiol. Additionally, a selective thiolation of carbon nanotubes is the synthesis of CNT—CH2—SH, which involves sonication of carbon nanotubes in nitric acid/sulphuric acid mixture for 24 hour to introduce carboxylic groups at the surface of CNTs, and treatment of carboxylic CNTs with sodium borohydride to split carboxylic group into alcoholic groups. The synthesis of CNT—CH2—SH also involves conversion of alcoholic groups to the corresponding chloride groups, and finally treatment of the chloromethylated carbon nanotubes into mercapto methylated CNTs during the reaction with thiourea. The single wall carbon nanotubes can also be functionalized with cysteamine molecules.
In the majority of prior art, the link between carboxylic nanotubes and amine group in cysteamine can be performed using a carbodiimide as a catalyst, (i.e. 1-ethyl-1,3-[3 dimethylaminopropyl]carbodiimide hydrochloride). Recently, conductive organic polymers can be used to prepare organic compounds based sensing films with versatile applications such as gas sensors, solar cells, batteries, antistatic coatings, electro-luminescent devices, electrodes, nonlinear optical devices, transistors, etc. Polythiophene (PT) is an organic polymer, which exhibits high environmental stability, facile routes of chemical or electrochemical synthesis and functionalization, and thermal stability. A variety of derivatives of polythiophenes can be synthesized in the form of poly(3-butylthiophene), poly(3-hexylthiophene), poly(3-octylthiophene), poly(3-thiophenealkanesulfonate), poly(3-dodecylthiophene), and poly(3-perfluorooctylthiophene). Such polythiophene can be functionalized with macrocyclic cavities of crown ethers for detection of metal ions. The polythiophene is also functionalized with chiral primary amine for separation of chiral species, especially chiral amino alcohols. Recent publications relate the synthesis of poly[3-(6-bromohexylthiophene)] and poly[3-(12-bromododecylthiophene)], two types of poly[3(ω-bromoalkylthiophene)] which offer a myriad of ways for new functionalizations through the intermediate of displacement reactions. Synthesis of a novel compound, thieno[n]acene, was also reported. Composite polythiophene/boron trifluoride etherate was used to prepare sensitive film for detection of the following vapors: n-hexane, ammonia, triethylamine, acetone, water, trimethylamine, toluene, alcohols. The sensitivity response is proportional with the polarity of tested vapors. Even such combinations of the polythiophene and the carbon nanotubes often lack in H2S sensitivity, and mechanical and electrical properties. Hence, it is desirable to manufacture miniaturized solid-state sensors with increased performance for H2S sensing.
A need therefore exists for an improved method for the design and preparation of a matrix nanocomposite-based sensing film with high sensitivity, which enables hydrogen sulphide SAW/BAW detection at room temperature. Such an improved method is described in greater detail herein.